Resin impregnated materials, commonly referred to as "prepreg" materials, are in wide use as sheets and tapes, for example, for molding relatively thin walled articles of often intricate shapes. Typically, such materials are made by applying a resin, e.g. polymerizable thermosetting or a heat softened thermoplastic), to bands of parallel fibers or filaments (for tapes) or to fabrics (for sheets) made by matting, knitting, weaving etc. fibers which extend in multiple directions.
To assure high quality finished articles made of prepreg materials the resin must fully penetrate the band or fabric, i.e. it must contact substantially the entire surfaces of all fibers. In other words, to obtain satisfactory end products the resin must effectively encapsulate the fibers.
Prepreg materials are extensively used in the aerospace industry because they yield parts which are relatively light weight, have good strength and are inexpensive to form into intricately shaped articles. In the past, thermosetting resins were preferred over thermoplastic resins because the latter had relatively low softening temperatures even though they had a number of highly desireable characteristics such as, for example, their ability to be reheated to remake a previously defectively formed part, or their greater impact resistance.
Recently, the aerospace industry has shown a strong interest in prepreg materials which use so-called engineered thermoplastic resins. A common characteristic of these resins is that they have a long chain, linear molecular structure. These materials differ from conventional thermoplastics in that they have relatively high melting or glass transition temperatures.
The problem encountered when using long chain thermoplastic polymers in prepreg materials is their highly viscous nature which makes it virtually impossible to impregnate any significant thickness of fiber materials, such as the above mentioned bands, sheets or mats, with them. Increasing the pressure of the heat softened polymer does not help because the intertwined long molecule chains cannot pass between the fibers and penetrate into the fiber material. Instead, the intertwined long molecules form a "log jam" over the surface of the fiber material because they are randomly oriented. When the pressure is increased, the molecular log jam across the surface of the fiber material has the tendency to limit the pressure to the side where it is applied which leads to lateral movement of the polymer rather than impregnation.
As a result long chain polymers are not extensively used for impregnation using hot melt techniques for the manufacture of prepreg materials in spite of the many desirable characteristics they have.
The problems encountered when using long chain polymers for prepreg materials is well known. U.S. Pat. No. 4,559,262 (Cogswell), for example, mentions the desirability of using thermoplastic resins, notes that such resins which have acceptable physical properties require a high molecular weight (typically in excess of 100 Ns/m.sup.2) which renders the resin so viscous that the fibers cannot be adequately wetted, and then discloses that an acceptable compromise can be struck and prepregs having relatively desirable characteristics can be obtained with relatively low molecular weight thermoplastic resins which have a sufficiently low viscosity that adequate fiber wetting is possible. Of course, this prior art approach, though it may be an improvement over what occurred before, cannot make use of the much more desirable, high molecular weight and high viscosity thermoplastic resins because the process described in that patent is effectively limited to resins having a viscosity of no more than about 30 Ns/m.sup.2, i.e., low molecular weight thermoplastics. The present invention makes it possible to use high molecular weight, long molecular chain resins of a viscosity as high as several hundred Ns/m.sup.2 while assuring a thorough and complete wetting of the filaments and, therefore, a superior prepreg material.